Dynamic rotary seals are used in numerous engineering situations. One example in the aerospace industry is around actuator shafts where the shaft enters a gear box. The seal prevents water and other contaminants from entering into the gear box where these can cause wear and degradation of the gear box components and can lead to a decrease of efficiency and lubricant performances after a few years of service.
The durability requirements for aircraft components can be extremely demanding. For example, the mean time between failures (MTBF) for actuator shafts and seals can be very high, e.g. 250,000 flight hours. Such components are often expected to remain on the aircraft throughout the aircraft's entire life without replacement or major servicing (other than e.g. lubricant changes), thus a shaft/gear box seal might be expected to remain in service for 30 years in some cases.
As part of the seal arrangement, a flexible (e.g. elastomeric) seal component presses against the rotating shaft so as to create a barrier to ingress of contaminants. The materials from which such shafts are made (e.g. typically determined by required weight and/or torque characteristics) cannot provide sufficient wear resistance due to insufficient surface hardness. Over time, the contact and friction between the seal component and the shaft causes wear of the shaft. Too much wear of the shaft degrades the seal to the extent that contaminants can pass through the seal and the seal is considered to have failed.
The amount of wear experienced by the shaft depends on the force of the seal component against the shaft. Some seal designs result in more force than other seal designs and therefore result in more wear on the shaft.